I'm sitting in a huge sound-stage in Burbank, Calif., and I've got a serious case of the willies. First off, I still can't believe what I'm seeing: Brian Wilson, fronting a 10-piece band, poised to launch into a rehearsal of "Smile." This is so surreally unbelievable--like an acid flashback to something that never actually happened. In the heavily chronicled legend of Wilson and the Beach Boys--a story that begins in innocence and ends in drugs, mental illness and acrimony--"Smile" is the enigmatic centerpiece. It is the most famous pop-music album never released. Imagine that "Sgt. Pepper" hadn't come out but remained only a rumor. That's the story of "Smile."

Wilson started work on it in 1966, after completing "Pet Sounds." By then he had quit touring with the band and had become a total creature of the recording studio. With his lyricist, Van Dyke Parks, he labored for more than a year on what was to be an epic album-length musical meditation on America. Conceived and created just when Wilson was at the height of his contest with the Beatles for artistic and chart-topping supremacy, the three-part "Smile" was the album that would secure his place as the reigning king of popular music. Then, with 400,000 album covers already printed, Wilson pulled the plug, offering no explanation. A month later, the Beatles released "Sgt. Pepper" and rewrote rock history.

A handful of songs that were to have been part of "Smile"--"Good Vibrations," "Heroes and Villains"--trickled out on subsequent albums, enough to convince anyone who heard them that this had been the Masterpiece That Got Away, an emblem of the failed promise of the '60s. Then, about a year ago, as abruptly as he'd abandoned the project 36 years earlier, Wilson announced that he would finish "Smile." When I heard the news, I didn't know what to think. To those of us who have loved Wilson's music for 40 years, "Smile" would always be a masterpiece--as long as we never heard it. How could the real thing live up to the expectation? I'm about to find out, and I'm not sure I want to.

The day before, I'd interviewed Wilson at his Beverly Hills home, where the 62-year-old musician lives with his wife, Melinda, their three young children and 11 dogs. It began promisingly when he met me in the foyer, a big shambling man well over six feet with a full mane of graying hair and the most innocent yet saddest eyes I'd ever looked into. "Come on in my room," he said, leading the way up the curving staircase. Thirty minutes later, he was out of his chair and gone like a spring-loaded toy, and I was packing up, wondering what I was going to tell my boss.

I began by asking why he'd decided to finish "Smile" at last. "Because we thought it was appropriate music for the times now." And why so? "The lyrics express the mood of Americana." I waited for him to continue. He didn't. Suddenly what had seemed an embarrassingly long list of questions looked all too short. How was he different from the man who started "Smile" almost 40 years ago? "I'm more of a perfectionist about harmonies." I waited: that was it. This was the longest half hour of my life.

OK, so Wilson is nobody's idea of an average guy. Abused as a child, he's suffered from depression all his adult life, and he's a complete eccentric. Exhibit A: that piano he placed in a sandbox in his living room in the '60s. "Sometimes I wonder if Brian can tie his shoes," says Darian Sahanaja, one of the band's keyboardists and its musical secretary. "On the other hand, he remembers every phone number he's ever known." Everyone in his band insists--and a band that's stuck with a man for five years should know--that he has no meanness in him. And that he cannot tell a lie. When a certain famous rock star asked for an autograph on "Pet Sounds," Wilson wrote, "Thanks for all the great songs," thought for a moment and then crossed out "great" and wrote "good."

Wham! A downbeat from the drummer and the band charges into "Heroes and Villains" with everything it's got. The keynote song in a three-part suite, it rises and falls, changes tempo, elides into another melody, only to resurface again and again. By the time the band wraps up its migration through "Plymouth Rock" ("Rock, rock, roll, Plymouth Rock roll over") to "Cabinessence," I have almost forgotten the rocky interview from the day before. But I do remember asking Wilson if he wouldn't feel more comfortable if we sat at his grand piano and conversed in chords. "Yes," he quickly replied. "Absolutely."

Music has always been the polestar of Wilson's life, ever since he fell in love with "Rhapsody in Blue" at the age of 2. He crafted dozens of hit songs for the Beach Boys, the band he'd formed with his brothers Carl and Dennis, before he was 25, and while his output has slowed a lot, he's still at it. "Gettin' In Over My Head," a new solo album, appeared last month. True, except for a couple of brilliant songs this new music sticks to the shallow end of the pool. But the work Wilson has done lately on "Smile" dramatically proves he hasn't lost his touch. Taking what were glittering musical fragments, he completely resequenced and rerecorded the work from scratch, seamlessly adding new music and lyrics, especially in the third, least finished section. When I asked what he does for fun, music was the only thing he mentioned. "I go to the keyboard and I write and play two hours a day," he said. And every night he lets his daughter drag him upstairs to sing "Barbara Ann" and "Surfer Girl" before bed.

"Smile's" second movement climaxes with "Surf's Up," a baroque hymn to the lost innocence of a generation and, beyond that, of the nation as a whole. The melancholy is oblique ("Columnated ruins domino") but ultimately, as sung in Wilson's weathered falsetto, downright heartbreaking ("A choke of grief, hard hardened I/ Beyond belief, a broken man too tough to cry"). For this, much of the credit belongs to the lyricist Parks, who left the original project after the other Beach Boys reviled his work. "Smile" may be Wilson's belated triumph, but it is Parks's vindication.

Because "Smile" is not merely a collection of songs but a through-written piece that demands shaping, it helped that Parks is also a classically trained musician (who produced his own album-length masterpiece, the 1968 cult favorite "Song Cycle"). "What we wanted to do," Parks says, "was to make some condensate of thought about Manifest Destiny, from the moment the Pilgrims came onto Plymouth Rock--warts and all. And in finding a peculiarly American pop-music experience in that, we were successful even in the estimation of Leonard Bernstein, who called 'Surf's Up' 'an important contribution to 20th-century American music'."

Bernstein, schmernstein. This music will still be fun, fun, fun long after Daddy takes the T-bird away. At that rehearsal I realize for the first time just how funny this music is, with the musicians doing their best Spike Jones imitations with power drills and saws and those little cans that moooo when you turn them upside down. That's "Smile's" secret weapon--its blend of silliness and seriousness, slide whistles and French horns. That's what makes it, in Sahanaja's perfect phrase, a thing of "childlike complexity."

Midway through the rehearsal, Melinda Wilson leans over to me and says, "It wasn't until we got to London [for the premiere in February] that Brian admitted to me that 'Smile' was the best stuff he ever did. Imagine having your best stuff bottled up like that for 37 years." I think I know how he felt, just a little, because when "Good Vibrations" caps the closing movement at the end of the rehearsal, I let out my own sigh of relief. He finally pulled it off, I'm thinking. Four decades is a long time to wait for anything, but in this case it was worth every minute. With the last chord still hanging in the air, I watch as Wilson bolts for the door, hurrying to get to the dentist to get his teeth whitened--more smile work. Me, I'm in no hurry. Right now I wouldn't trade places with anybody on the planet.